Unbend object bent with simple deform modifier
You can try this way:

Let's say you have object A (your half cylinder). Create a plane (object B), give it a Simple Deform/Bend modifier, bend it 180°:




Overlap the 2 objects (scale object B if necessary), give to object A a Surface Deform modifier with object B as Target, click on Bind:



Bring back object B to  an angle of 0°, object A will follow: